Cellular Trends Steer Process Technology Advances

In addition to raising integration and performance in cellular handheld devices and infrastructure, semiconductor processes are supporting opportunities in alternative-energy and broadcast applications.

Debates often rage in this industry over which semiconductor process is better for a given use. although some applications are better served by a particular process technology, there is room for many. a wealth of choices are offered by process technologies like CMos, biCMos, gallium arsenide (gaas), indium phosphide (inP), gallium nitride (gan), silicon carbide (siC), silicon bipolar, silicon-on-insulator (soi), and silicon-on-sapphire (sos). although semiconductor devices continue to employ higher levels of integration, discrete devices also are fabricated with these processes. With telecommunications networks around the world marching toward fourth-generation (4g) technologies like Long Term Evolution (LTE), discrete approaches are becoming a rarity in the communications arena. in fact, many believe that smartphone adoption will sound the final death knell for discrete implementations in handheld-communications products.

With a handheld device like an iPhone, for instance, consumer demands like more battery time, more multimedia features, and small phone size drive product design. Providing more features and functions with less chips means savings in space and cost. In addition to housing extra components, today's semiconductors and integrated circuits (ICs) must provide lower power consumption, ease of design, affordability, and more. This same trend is being seen on the infrastructure side, as network providers look to lower power consumption with "green" base stations while handling an increasing data load.

Proof of this trend can be seen at TriQuint Semiconductor, which is now promoting its more integrated versus discrete solutions. "The excellent high-power performance of GaNon-SiC is well-recognized in the industry," says Mike Peters, the firm's Director of Marketing for Commercial Foundry. "TriQuint's approach is to offer a full MMIC GaN process, which allows more complete integration within these higher-power applications. Also, our TriPower family of RFICs is offering new performance benchmarks in the industry. Operating in a systemic Doherty configuration, two TriQuint TG2H214120-FL 120-W devices can deliver over 60 W of average WCDMA power with 55-percent collector efficiency. The TriPower devices are also easily linearized with conventional digital-pre-distortion (DPD) techniques."

To handle the demands associated with the increase in bandwidth and modulation complexity of the evolving 4G standards, as well as the current build-out of 3G standards, Peters notes that future process-technology offerings will be required. "These will involve increasing integration, increasing power and efficiency, and lowering system costs," he predicts. "The technology improvements will be incorporated into the compound semiconductors (GaAs and GaN) as well as packaging techniques like Cu flip-chip."

Process-technology developments are indeed being fueled to push the evolution of cellular communications. Last May, for example, Peregrine Semiconductor Corp. made headlines by partnering with IBM Corp. When fully qualified, Peregrine's next-generation UltraCMOS RF ICs will be manufactured by IBM in a jointly developed, 180-nm process at IBM's 200-mm-wafer semiconductor manufacturing facility in Burlington, VT. More recently, Peregrine teamed with Soitec, a provider of silicon-oninsulator (SOI) wafers, to develop a bonded, silicon-on-sapphire substrate for RF IC manufacturing. The development and ramp-in production of the new substrate has been qualified for use in manufacturing Peregrine's next-generation STeP5 UltraCMOS RF ICs.

The firms were able to transfer and bond a monocrystalline thin-silicon layer onto a sapphire substrate. The resulting bonded silicon layer offers improvements in transistor mobility and silicon quality beyond conventional SOS wafers, which utilize an epitaxially grown silicon layer.

For Peregrine, the new substrate is expected to enable enhancements in RF IC performance, functionality, and form factor. It can supposedly enable IC size reduction and increase performance by as much as 30 percent. The substrate also will help Peregrine continue its long-term strategy toward highly integrated RF front-end (RFFE) IC solutions in a substrate technology that matches the yield and scalability qualities of bulk silicon technologies.

In other development news, Cree, Inc. has been making major strides in SiC technology (Fig. 1). Last August, the firm demonstrated high-quality, 150-mm SiC substrates with micropipe densities of less than 10 micropipes/cm2. Currently, Cree uses 100-mm-diameter SiC substrates. SiC fabricates products for a broad range of lighting, power, and communication components including RF power transistors for wireless communications. The availability of 150-mm SiC substrates provides increased throughput with a corresponding cost reduction.

SiC is often competing with rival process GaN. In addition to communications applications, for instance, GaN has gained acceptance in fields like alternative energy. According to RFMD's Joe Johnson, VP of MPG Advance Engineering, and Todd Gillenwater, VP of CPG Technology Platforms, "GaN has the highest power density of any semiconductor material5 to 10 times that of silicon or GaAs and twice the power density of SiC. For RF applications, high power density means the device can be very small with very low parasitic capacitances, giving very large bandwidth and high input/output impedances. GaN material also has an extremely high critical field, meaning high breakdown voltages allowing base stations to operate at much higher voltage, translating to overall better system efficiency. "Other applications adopting GaN include high power electronics, such as converters/inverters and motor drives powering for hybrid vehicles, and various industrial applications. High efficiency offered by GaN products make it ideal for tying photovoltaic and wind power systems to the power grid. GaN enables faster switching speeds with lower power loss in power-electronics components. The ability to reduce the power loss in grid components by as much as 30 percent makes GaN a truly green technology.'"

Because it is a relatively immature technology, Johnson and Gillenwater note that GaN is still relatively expensive. But the cost is coming down quickly as larger-diameter substrates become available and yield higher production volumes. The firm's GaN technology has now been in production for two and a half years. In its CATV amplifiers, GaN is used for HFC networks to extend the range of signal transmission from the head-end to the consumer. Compared to GaAs amplifiers, RFMD claims that these amplifiers provide higher output. In addition, the RF393x family of GaN unmatched power transistors beat out both GaAs and silicon in output-power performance.

GaAs continues to provide key advantages, however. For example, Avago Technologies recently leveraged its 0.25-m GaAs enhancement-mode pHEMT semiconductor process to create the MGA-31589 and MGA-31689 gainblock power amplifiers (PAs). By delivering high gain, they promise to reduce the total number of RF stages needed in wirelessinfrastructure applications (Fig. 2). In addition, a series of RF/IF variable-gain amplifiers (VGAs) from Analog Devices (www. analog.com) leverages both GaAs and SiGe to better serve base stations, industrial/ instrumentation, and defense equipment. This series of RF/IF VGAsdubbed the ADL5201, ADL5202, ADL5240, and ADL5243combine up to four discrete RF/ IF blocks into a single device. The ADL5201 and its dual version, the ADL5202, are digitally controlled IF VGAs that are designed to support high-IF-sampling receiver designs. (See "ICs Control Gain In Wireless Networks," Oct. 2010, p. 94.)

Mobile Devices Pack it in In addition to integration, performance, and power requirements, mobile devices must balance a barrage of standards and technologies. Examples include Bluetooth, wireless-local-area networking (WLAN), Global Positioning Satellite (GPS), and varied cellular standards. With smartphones and tablets growing in popularity, this demand is only going to rise.

Among the many examples of products that answer this trend is Maxim Integrated Products' MAX2667/ MAX2669. As the newest addition to the firm's GPS/GNSS low-noise-amplifier (LNA) family, these devices leverage SiGe processes to improve GPS receiver sensitivity in smartphones, personal navigation devices, and other handheld devices. Compared to discrete or highly integrated CMOS solutions, the LNAs promise to improve receive sensitivity and read range with a noise figure of only 0.65 dB. To complete a boardlevel design, both LNAs demand only four external components (plus an optional resistor for logic-enabled shutdown). They are housed in a 1 mm2 WLP.

Specifically targeting smartphones and tablet PCs, RFMD has introduced a number of SOI-based switch products. Due to the co-existence requirements of multiple radio standards (GSM/WCDMA/LTE/WiFi/ Bluetooth) in these mobile devices, they promise to provide leading switch linearity and harmonic performance. The portfolio of SOI switch products includes the RF1603A (SP3T), RF1604 (SP4T), and the RF1291 (SP10T) antenna switch module.

In a single package, a family of surfacemount PA modules from Skyworks, Inc. offers full wideband code division multiple access (WCDMA) coverage over the following ranges: 1920 to 1980 MHz (SKY77701), 1850 to 1910 MHz (SKY77702), 1710 to 1785 MHz (SKY77703), 824 to 849 MHz (SKY77704), and 880 to 915 MHz (SKY77705). The devices promise to meet the stringent spectral linearity requirements of high speed downlink packet access (HSDPA), high speed uplink packet access (HSUPA), and Long Term Evolution (LTE) data transmission with high power-added efficiency (PAE). A directional coupler is integrated into the modules, eliminating the need for any external coupler. The PAs are leveraged in smartphones from Taiwan's HTC including the EVO, Desire HD, and Z.

Clearly, the rate at which process technologies are advancing to meet the needs of wireless and other applications is simply staggering. Although integrated approaches are more often becoming the favored choice, discrete approaches will still be needed in applications beyond smartphones. As RFMD's Johnson and Gillenwater summarize, "Wireless applications use a variety of process technologies today: GaAs HBT, pHEMT, BiFET, SiGe, SOI, and CMOS. The most demanding applications in terms of performance will continue to use compound semiconductors. Where performance is less demanding, Si can be used. As the cost of GaAs solutions continues to come down (die shrink, higher volume), there is no compelling reason to change technologies.

"Interesting new technologies to watch over the next several years will be BiFET and SOI," they continue. "SOI is a relatively new Si technology for wireless applications and has some interesting RF characteristics that make it a good solution for low-power RF circuits and switches. GaAs BiFET combines HBT and pHEMT into a single technologyallowing higher levels of integration without sacrificing performance, and while reducing cost. LNAs, medium-power RF switches, HBT power amplifiers, and low-density analog control circuits can be included on a single GaAs substrate." Certainly, the debates over which application is best served by which process technology will continue to rage going forward.